Oct. 7, 2010 -- Nearly 50 million Americans have doctor-diagnosed arthritis, and 21 million people say the disease limits their physical activities, the CDC says.

In its Morbidity and Mortality Weekly Report for Oct. 8, the CDC says arthritis is increasing, that it's especially common among people who are obese, and that unless Americans learn to control their weight, the prevalence of the disease is sure to keep rising.

Arthritis represents a major public health problem in the United States "that can be addressed, at least in part, by implementing proven obesity prevention strategies and increasing availability of effective physical activity programs and self-management education courses in local communities," study authors write.

Among major findings from the National Health Interview Survey for 2007-2009:

22.2% of adults aged 18 and older say doctors have diagnosed them with arthritis. That's 49.9 million people.

42.4% of people with diagnosed arthritis, or 21.1 million people, said they suffer limitations on physical activity because of their disease.

Among the obese, 33.8% of women and 25.2% of men told interviewers they had been diagnosed with arthritis by a physician. Those rates were close to double that of people who are underweight or normal weight, 13.8% for men and 18.9% for women.

The prevalence of arthritis increases significantly with age and risk is affected by educational attainment, weight, physical activity, and lifestyle factors such as smoking, the authors say. They also report that arthritis-associated activity limitations are on the rise, fueled by the aging of the population as well as increasing obesity rates.

"With the aging population and continued high prevalence of obesity," arthritis is predicted to increase significantly over the next 20 years, the report says.

It's expected that the number of adults with arthritis will hit 51.9 million in 2010 and 67 million by 2030.

Other key findings of the study:

24.3% of women surveyed had doctor-diagnosed arthritis, compared to 18.2% of men.

21.9% of people with less than a high school diploma had arthritis, vs. 20.5% of people reporting they'd spent at least some time in college.

16.9% of normal or underweight people had been diagnosed with arthritis, compared with 19.8% of obese people.

23.5% of physically inactive people had arthritis, vs. 18.7% of those who reported that they engaged in recommended levels of exercise.

23.7% of current smokers and 25.4% of former smokers said they'd been diagnosed with arthritis, compared with 19% who had never smoked.

People who were surveyed were asked if they had been diagnosed with some form of arthritis, rheumatoid arthritis, gout, lupus, or fibromyalgia by a doctor or other health professional. The report says arthritis results in costs of $128 billion a year and is the most common cause of disability.

The lifetime risk for diagnosis with knee osteoarthritis is 60.5% among people who are obese, double that for normal and underweight people.

The researchers say major efforts are needed to reduce obesity, because even a small weight loss (about 11 pounds) can reduce the risk for knee osteoarthritis among obese women by 50% and could cut mortality risk by half.